The world currently consumes around 83 million barrels of oil each day. This figure is projected to reach around 120 million barrels per day by 2010. It is also estimated that the world production of oil will begin to decline at some point in the next 30 years. The increased demand and slowing of production will begin to generate an oil shortage.
Once the point is reached where supply cannot meet demand market forces will dictate that the price will rise. Some estimates claim the price of oil may double every five years after the point of peak production. The inevitable price increase makes the use of biomass as an alternative energy particularly attractive. The abundance of biomass is sufficient to offset a significant fraction of the worlds current energy needs. Biomass can be used directly through combustion to produce heat and power. However, there remain issues relating to distribution of the biomass, local handling of the biomass and the combustion issues such as long startup times of such systems. One way to avoid such problems is to convert the biomass into a liquid fuel. This approach also has the tremendous advantage that the current fuel distribution infrastructure can still be utilized. Suitable liquid fuels include methanol, ethanol, dimethylether (DME) and FT syncrude. The biomass is converted to the fuel at a central facility and the liquid product is distributed via the current fuel distribution network.
The most common process to convert biomass into liquid fuels consists of two steps. In the first the biomass is converted into a mixture of carbon monoxide and hydrogen. The process is called gasification and the gaseous mixture produced is often referred to as “Syngas”. Biomass gasification can be broadly summarized as:CH1.4O0.6+0.2O2=CO+0.7H2  (1)However an energy balance across the Equation 1 reveals that the products contain more energy than the reactants, hence some of the biomass is burnt to offset this imbalance. Equation 1 illustrates how the typical ration of carbon monoxide to hydrogen is close to 1:1.
In the second stage the carbon monoxide and hydrogen are compressed and passed over a suitable catalyst. The reactions which convert the syngas to liquid fuels are exothermic so the reactor in which the process occurs has to have suitable facilities to remove this heat of reaction. The reactions producing methanol, ethanol, dimethylether and FT syncrude are shown below:2H2+CO→CH3OH  (2)4H2+2CO→CH3CH2OH+H2O  (3)4H2+2CO→CH3OCH3+H2O  (4)2nH2+nCO→n-CH2-+nH2O  (5)
It is interesting to note that for all of the reactions a hydrogen to carbon monoxide ratio of 2:1 is stoichiometrically required. If the syngas produced through direct gasification (i.e., with a CO to H2 ratio close to 1:1) is used directly the reaction will not proceed to completeness as hydrogen will become limiting. At best this result in a yield loss; however, many of catalysts used in the transformations are likely to suffer from carbon deposition resulting in reduced activity. To maximize the yield from the process it is important that the hydrogen to carbon monoxide ratio be controlled to match the stoichiometry of the liquefaction process. One method in which this can be completed is via the water gas shift reaction:CO+H2O=CO2+H2  (6)in which CO can be stoichiometrically interchanged with H2. This approach has been successfully used; however, the approach inevitably leads to a substantially mass loss of reactants, as carbon monoxide with a weight of 28 g/mol is used to produce hydrogen with a weight of 2 g/mol.
A number of US patents have been directed to apparatus suitable for the gasification of biomass. U.S. Pat. No. 4,583,992 issued to C. D. Rogers describes a gasification apparatus consisting essentially of an upright cylindrical downdraft gasifier upon which the gasification material is supported upon a rotational grate. The gasification apparatus is continuously fed into the vessel through an aperture situated on the top of the apparatus. Combustion air is supplied through a central pipe, originating at the top of gasifier, and which has outlets at various locations within the bed. This central pipe further proceeds through the bed and is attached to the grate. The central pipe is rotated to allow means for the rotation of the bed grate. Through the control of the rate of rotation of the grate the fraction of material exiting the system in the form of activated carbon is controlled. Rogers does describe a system suitable for the gasification of biomass and for the production of charcoal but does not teach of a method in which the composition of the outlet stream can be controlled.
U.S. Pat. No. 4,306,506 issued to F. Rotter describes a gasification apparatus consisting of a vertical cylindrical downdraft gasifier, in which the lower section is of double shell construction. In the inner section gasification processes, consisting of drying, distillation, oxidation and reduction occur. The bottom of the gasification section is comprised of a cone such that the local superficial velocity is increased to aid in heat and mass transfer. After passing through a grate the syngas is passed in an upward direction through the annulus created by the double shell construction. Here heat is transferred from the syngas exiting the system to the gasification processes occurring in the inner section, thus improving the thermal efficiency of the process and avoiding the need for an external heat exchange device. The apparatus is designed such that the inner section is hanging within the outer section and thus allowed to thermally expand as desired. This arrangement is said to prevent the build up of harmful stresses with in the structure. The patent issued to Rotter does teach of a downdraft gasifier design in which heat is recovered within the gasifier apparatus but the patent docs not teach of a method in which both oxygen and steam and introduced through a multi-injection array to control the processes occurring within the gasifier.
U.S. Pat. No. 4,929,254 issued to C. A. Kooiman relates to a gasification system for the production of a clean combustible products form solid fuel material. The apparatus is a down draft gasifier and consists of vertically orientated apparatus having an uppermost portion which comprises a hopper for the delivery of feedstock. An air tight locking device is used to separate the top of the gasifier from the feed hopper. The vertical chambers of the gasifier comprise a first drying chamber and intermediate and lower gasification chambers. The chambers are modular units which vertically align. Air inlets are present in the intermediate chamber through which the oxidant is introduced. At the bottom of the lower chamber a grate is located to support the bed. An outlet is also located within this bottom section such that the process gas can be removed from the apparatus. Kooiman also teaches of a number of external operations, consisting of quenchers, scrubbers and filters which lead to the production of a clean syngas. The patent does not teach of a system in which oxidants and steam are injected below the grate arrangement to promote the oxidation and reformation of tars and low order hydrocarbons.
U.S. Pat. No. 4,004,896 issued to S. L. Soo teaches of a method of steam reforming of a carbon containing feedstock. In the method excess steam is generated in a packed superheater before being contacted with the carbon containing feed. The patent teaches that by using steam in excess by a factor of 2-10 that all of the heat of reaction of steam gasification can be supplied by the sensible heat contained in the steam. Furthermore the patent teaches that by the careful control of the amount of the steam that the composition of the output ratio of carbon monoxide to hydrogen can be controlled. Through the use of a large excess of steam a stream composed of 90% hydrogen can be produced. The technique is applicable to both batch and continuous processes. However, the patent does not teach how through the use of combinations of steam and oxygen as oxidants the syngas composition can be controlled without the need of a large excess of steam.
U.S. Patent Application No. 2004/0013605 applied by Ramani et al. teaches of a method to control the ratio of carbon monoxide to hydrogen in a syngas by the reformation of hydrocarbons. The patent teaches of a method where two feedstocks are selected, where the first feedstock, upon reformation, gives a higher H2:CO ratio than desired and the second feedstock, upon reformation, gives a lower H2:CO ratio than desired. The patent teaches how the ratio of the two feedstocks can be calculated such that the combination of products gives the ratio desired. The patent teaches that the fuels can be either combined and reformed together or the reformation carried out separately and the products of the reformations combined. The patent does not teach how the composition of a syngas stream produced from a biomass feedstock can be controlled by the combination of oxidation and steam reforming processes.
It is therefore a feature of the current invention to provide a method and apparatus for the gasification of biomass in which the ratio of carbon monoxide to hydrogen in the outlet gas is controlled. The method utilizes a combination of oxygen and water as the oxidant. The oxidants are injected at a number of locations to control the extent of oxidation and reformation processes occurring within the gasifier.
It is a further feature of the present invention to provide a design of a downdraft gasifier in which the gasifier is separated in a top section and bottom section by a grate. The grate is used to support biomass such that a bed is formed. In the top section oxygen and steam are injected into the biomass to initiate the drying, distillation, oxidative and reduction of biomass and biomass products. Just above the grate a bed of embers exist where significant thermal cracking and steam reforming reactions act to break down any tars produced during the pyrolysis of the biomass. Below the grate a second set of oxidant injection nozzles are present. The introduction of oxygen and steam promote a second reaction zone which allows adjustment of the syngas composition and acts to destroy any tars and low order hydrocarbons, such as methane, via a second oxidative and reformation step. The technique produces a syngas which is low in tars and methane and which maximizes the carbon monoxide and hydrogen yield from biomass.